300 



EARTH FEATURES AND THEIR MEANING 



of such marks to the southward of the common border. Within 

 the American glaciated region there is, however, an area surrounded 

 like an island, and within this district (Fig. 324) none of the marks 

 characteristic of glaciation are to be found. This area is usually 

 referred to as the " driftless area," and occupies portions of the 

 states of Wisconsin, Illinois, Minnesota, and Iowa. Even better 

 than the area to the southward of the Ohio and Missouri rivers, it 

 permits of a comparison of the nonglaciated with the drift-cov- 

 ered region. 



The " driftless area." Within this district, then, we have 

 preserved for our study a landscape which remains largely as it was 



before the several ice 

 invasions had so pro- 

 foundly transformed the 

 general surface of the 

 surrounding country. 

 Speaking broadly, we 

 may say that it rep- 

 resents an uplifted and 

 in part dissected plain, 

 which to the south and 

 east particularly reveals 

 the character of nearly 

 mature river erosion 

 (Fig. 177, p. 170). The 

 rock surface is here 

 everywhere mantled by 

 decomposed and disin- 

 tegrated rock residues 

 of local origin. The 

 soluble constituents of 

 the rock, such as the 

 carbonates, have been 

 removed by the process of leaching, so that the clays no longer 

 effervesce when treated with dilute mineral acid. 



Wherever favored by joints and by an alternation of harder 

 and softer rock layers, picturesque unstable erosion remnants or 

 " chimneys " may stand out in relief (Fig. 326). Furthermore, the 

 driftless area is throughout perfectly drained it is without lakes 



FIG. 320. "Stand Rock" near the "Dells" of the 

 Wisconsin river, an unstable erosion remnant char- 

 acteristic of the driftless area of North America 

 (after Salisbury and Atwood). 



